The general process of manufacturing metal-semiconductor field effect transistors (MESFETS) is well known. Source and drain regions are formed in a substrate by an N-type impurity which is implanted with a sufficient dose to provide N-plus conductivity, and a channel region is provided between the source and drain regions with an N-type impurity implanted with a lower concentration, sufficient to provide N-minus conductivity. Ohmic contacts are made to the source and drain regions. The channel region is then selectively etched, removing the substrate in a portion of the channel region, and a gate electrode is deposited into the resulting recess.
The threshold voltage of an enhancement-mode MESFET, i.e., the voltage which must be applied to the gate electrode in order to permit current flow between source and drain, is a function of the distance between the bottom of the gate recess and the boundary between the channel region and the substrate, which distance is referred to herein as the "channel depth". The pinch-off voltage of a depletion-mode MESFET, i.e., the voltage which must be applied to the gate electrode in order to cut off the current flow, is also a function of channel depth. The threshold voltage of the enhancement-mode (E-mode) MESFET and the pinch-off voltage of a depletion-mode (D-mode) MESFET are generically referred to hereinafter as the "critical gate voltage". The magnitude of the critical gate voltage of an enhancement-mode MESFET is considerably less than that of a depletion mode MESFET. But for a constant, the critical gate voltage of each type of MESFET is proportional to the product of the impurity concentration in the channel region and the square of the channel depth. Therefore, if the channel depth of an enhancement mode MESFET is to be the same as that of a depletion mode MESFET, it is necessary that the concentrations of impurity in the gate regions of the respective MESFETs be different. This implies two separate channel implantation operations, and use of two different masks to define the regions to be implanted.
It is known that compound semiconductor materials are anisotropic, and etch at different rates in different directions.